A far-infrared Fabry—Pérot interferometer and grating spectrometer for balloon-borne astronomy

Abstract A far-infrared computer-controlled Fabry—Perot interferometer has been built for use on the UCL 60 cm stabilised balloon platform for the observation of fine structure and molecular lines at moderately high spectral resolution (λ/δλ $ 1000). The interferometer is combined with a helium cooled grating spectrometer which allows operation anywhere in the 40–115 μm wavelength range whilst minimising the thermal background radiation onto the detectors. This grating spectrometer can also be used on its own for low-resolution (1 cm −1 ) spectroscopy. The Fabry-Perot incorporates a novel scanning and capacitance micrometer servo control system which allows a large movement range (60 μm) whilst actively maintaining the etalon parallel to better than 0.5 μm. The etalon can be scanned at high speeds (typically 10 scans per second) allowing an on-source/off-source spectrum pair to be taken in much less than one second through use of the telescope chopping secondary. This has allowed atmospheric water vapour features to be very effectively subtracted from the source spectra.